The Foreign Office is warning Britons not to travel to mainland China, unless their journey is essential.
It comes as Britons in Hubei Province, at the centre of the coronavirus outbreak, prepare to be flown home as early as Thursday.
The virus has caused more than 100 deaths, spreading across China and to at least 16 other countries.
There have been no confirmed cases in the UK so far, with 97 people testing negative for the virus.
The FCO had already warned against all travel to Hubei Province, saying anyone there who could leave should do so.
Its new advice warns against "all but essential travel to the rest of mainland China (not including Hong Kong and Macao)".
It warns the Chinese government is imposing further restrictions on movement, adding: "It may become harder over the coming weeks for those who wish to leave China to do so."
The FCO is arranging to evacuate Britons from Wuhan and the surrounding Hubei province, urging those who want to leave to contact the British consulate before 11:00 local time on Wednesday.
Up to 300 British people are thought to be in the area, which is now under strict travel restrictions. Officials estimate as many as 200 of them will want to return to the UK.
One teacher in Wuhan, the capital city of the province, told the Press Association UK citizens were being given details of forthcoming flights - with some understood to be scheduled to fly back on Thursday morning.
'Cities could be closed at short notice'
Travel journalist Simon Calder told BBC News the FCO's change of advice was "extremely significant".
"My estimate is that there are around 10,000 holidaymakers in China at the moment and many thousands more who are on business or living as expatriates," he said.
"The Foreign Office is being clear that it is not overly concerned with people who are there at the moment, it is more that they want to manage the scale of any possible problems.
"At the moment people can fly freely in and out of Beijing and Shanghai - the Foreign Office appears to be concerned that cities like that could be closed at short notice."
Mr Calder said airlines flying from the UK to China "have all said that they will provide refunds and anyone booked on a package holiday due to depart imminently will be entitled to a full refund".
Those with packages booked to depart beyond the next week or so may be told to wait and see by their travel company, in case the FCO changes its advice.
"Anyone who travels to China against the Foreign Office advice risks invalidating their insurance," he added.
Amie Keeley, head of news at Travel Weekly, said it was a quiet time for organised tours in China, but operators had been offering customers with bookings later in the year refunds or alternative travel dates.
Foreign Secretary Dominic Raab said the government was "working urgently to finalise arrangements for an assisted departure from Hubei Province for British nationals this week, and are in contact with people in Hubei to ensure they register their interest and that we can keep them updated".
Britons have also been advised by the FCO to "comply with any additional screening measures put in place by the local authorities".
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British Airways said it was assessing its schedule of flights to China, while Virgin Atlantic said its flights to Shanghai woud continue to operate as usual.
A UK expert developing a vaccine against the virus has said it has a "very good chance" of being effective.
Professor Robin Shattock, head of mucosal infection and immunity at Imperial College London, said he plans to start testing the vaccine on animals by the middle of February, with human studies in the summer if enough funding is secured.
Britons in parts of China affected by the virus have been criticising the UK government for not doing enough to help them leave.
Some in Wuhan said they had been given little information, compared with those from other countries. The US and Japan have been sending planes to Wuhan to evacuate their citizens.
Meanwhile, tests on people who have returned to the UK from the area are continuing.
One man currently being tested was filmed as he left home in Harborne, Birmingham, on Monday, on his way to hospital.
A video posted on Twitter by a neighbour showed him being escorted to an ambulance by a medic wearing a protective suit.
Earlier, Public Health England in the West Midlands said blood tests on the man were ongoing but it later said it was not in a position to comment on individual cases and would not provide rolling updates on tests.
It said testing for coronavirus involved taking samples from the patient's nose, throat and deeper respiratory tract and sending them to a laboratory which could provide a result within 24 hours.